The Flow team left Mingle in part because it was designer- and mockup-unfriendly. Trello lets you easily attach assets and one appears in card view.
- Phabricator mentions Pholio mocks, I'm not sure how that works.
- This edit window has an Upload file icon, tells you to drag and drop images... doesn't work in Firefox. It also has a weird "Meme" button.
- On Phabricator's site, you can see a task to which someone added Design, and then the task appears in a Design project view. Hmm